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Bearded capuchin monkeys at Fazenda Boa Vista, Piauí, Brazil older than 8 years routinely crack palm nuts with a stone hammer and anvil. An embodied, perception–action stance hypothesizes that monkeys' improving skill at this task is enabled by learning affordances of spatial and force relations produced with objects. Affordances are individual-relative opportunities for action that may change across development. We evaluated predictions linking behaviours with nuts and stones and body mass to success at nut cracking in young bearded capuchins (1.3–6.3 years old) and adults over three annual observation periods. Behaviours with nuts and stones reflected the monkeys' learning affordances for cracking; body mass constrained affordances for cracking. For each period in which they were observed, individuals were assigned to novice, intermediate or expert classes in accord with their success at cracking nuts. Novices did not crack nuts, despite striking them with stones. Body mass was modestly associated with intermediates', but not experts', likelihood of cracking a nut. Experts exhibited higher proportions of manipulation devoted to positioning nuts on the anvil and shorter bouts of cracking than intermediates; intermediates exhibited greater variability in these variables than experts or novices. The strongest classification model predicting individuals' assignments to skill class with 89% accuracy combined average bout duration and the ratio of manipulation devoted to positioning to other manipulation. The findings suggest practise using the body-pus-tool system for cracking palm nuts supports affordance learning and results in gradual mastery of this skill and that changing body mass plays a small role in this process. • Most monkeys <7 years old did not have adult (expert) skill at cracking nuts. • Body mass had little to no effect on monkeys' skill at cracking nuts. • Experts cracked nuts in short bouts and relatively frequently positioned the nut. • Less skilled monkeys used long bouts and varied duration and action more over bouts. • Monkeys slowly learn affordances of the body-plus-tool system for nut cracking. [ABSTRACT FROM AUTHOR]

Developmental psychologists have noted a similar timeline of change for children's use of different perspectives about the same objects or events, as in the use of different labels for the same object, an aspect of language, and in understanding other's knowledge or beliefs, an aspect of social cognition as reviewed in the study by Neiworth et al. Comparative psychologists are interested to know what cognitive flexibility looks like in other species and how such variation relates to life history, ecology, and phylogeny. The general pattern of results to date indicates that monkeys can master both intra- and interdimensional shifts, but intradimensional shifts are learned far more quickly than interdimensional shifts (reviewed in the study by Neiworth et al, 2022). Neiworth et al. report that they have conducted exactly this kind of comparative study: They examined cognitive flexibility in adult cotton-top tamarins and human children in three age groups as they participated in a modified version of the Dimensional Change Card Sort (DCCS). Neiworth et al.'s study offers an example of careful consideration of one such possibility: that of using the experimenter's postural orientation to the cards as an inadvertent aid. Here the authors had the benefit of prior work showing that tamarins follow human-provided cues to make a spatially discriminated choice only if the experimenter's head, body, and eyes oriented to a particular location. Thus, in this study, the experimenter kept their head and body centered in the testing space between the two cards and looked at a point on the wall directly behind the midpoint of the testing tray. But the DCCS task, in abstract form, has potentially broader comparative value than to examine cognitive flexibility in primates alone. [ABSTRACT FROM AUTHOR]

A scientific discipline grows through the insights and labors of individual scientists, honed by their discussions among colleagues and the mentoring they provide to the next generation of scientists. Margaret Floy Washburn, president of the American Psychological Association in 1921, the founding year of the Journal of Comparative Psychology, was a large presence during the early years of comparative psychology. She was a consummate scientist in all the abovementioned dimensions: insights, labors, communicating with her peers (including, a century later, readers of her voluminous writings), and mentoring. This essay provides an overview of her professional life and, more importantly, a synopsis of her major theoretical work, Movement and Mental Imagery, published in 1916. Her theoretical insights are remarkably relevant to contemporary developments in comparative psychology and related subdisciplines in psychology. She is an admirable founding mother for the discipline. [ABSTRACT FROM AUTHOR]

In this issue's featured article, Mercado and Perazio (2021) describe songs of humpback whales using acoustic qualities, in part to determine the degree of similarity in songs across time and space that have not been revealed by information-theoretic analyses. They are particularly interested in evaluating alternative explanations of song variations in humpback whales. They argue that if humpback whales' songs "are . . . transmitted through acoustic contact followed by imitation" (p. 29) then (a) songs of populations not in acoustic contact should diverge, (b) songs of the same population should diverge increasingly over time, and (c) song forms separated by multiple decades either within or across populations should be dissimilar. Alternatively, acoustic similarities in song structure across populations and/or across decades in the same population would challenge the hypothesis that socially mediated learning is the primary driver of variation in the structure of humpback whales' songs over time. If that is the case, then identifying universal properties of song composition is important to move the field forward. Broadening the analytical tool kit can help in this effort. [ABSTRACT FROM AUTHOR]

Tecwyn and Buchsbaum (2018) in this issue challenge the idea that dogs display a persistent gravity bias. In four experiments, they probed where dogs search for a ball after it is dropped into a tube. First, they replicated diagonal tube task experiments previously conducted with apes (Cacchione & Call, 2010) to investigate how auditory and visual information about the tubes influenced the dogs' search. Next, they examined how dogs' search shifted when the middle location was no longer an option. Third, they probed whether dogs' search could be explained by proximity between the reward's release point and the search locations (a condition depicted in Figure 1). Finally, they presented tasks in which any of the biases that might have guided them in the previous experiments (namely, gravity, middle position, or proximity to the point where the ball was dropped) could not guide their search. [ABSTRACT FROM AUTHOR]

Comments on an article by W. T. Herbranson et al. (see record 2022-07304-001). The article by Herbranson et al. illustrates the care that must be taken, both in designing comparative research and in interpreting the findings, to understand how specific features of experimental design impact each species' choices. Herbranson et al. presented to pigeons a decision-making challenge known informally as "The Secretary Problem", a problem cast in the form of selecting a candidate for a job. The task is to make a single choice among a finite set of options when these options are presented in succession. The chooser must select a given option or pass on to the next without knowing what the remaining options are. Decisions are final--one either rejects an option when it appears or selects that option, ending the search. The authors trained pigeons to recognize the "reward value" of five different colors on keys that they could peck. Herbranson et al. subsequently replicated the study with humans, presenting humans with probabilistic outcomes (rather than the certain outcomes of varying value presented in earlier studies with humans). Their aim was to explore the consequences of altering reinforcement options on humans' choices, so as to understand more fully the ways in which pigeons and humans approach this problem. Herbranson et al.'s work is an example of the power of carefully constructed comparative behavioral experiments to expand the understanding of ourselves and other species in unexpected ways. [ABSTRACT FROM AUTHOR]

Effective vision for action and effective management of concurrent spatial relations underlie skillful manipulation of objects, including hand tools, in humans. Children’s performance in object insertion tasks (fitting tasks) provides one index of the striking changes in the development of vision for action in early life. Fitting tasks also tap children’s ability to work with more than one feature of an object concurrently. We examine young children’s performance on fitting tasks in two and three dimensions and compare their performance with the previously reported performance of adult individuals of two species of nonhuman primates on similar tasks. Two, three, and four year-old children routinely aligned a bar-shaped stick and a cross-shaped stick but had difficulty aligning a tomahawk-shaped stick to a matching cut-out. Two year-olds were especially challenged by the tomahawk. Three and four year-olds occasionally held the stick several inches above the surface, comparing the stick to the surface visually, while trying to align it. The findings suggest asynchronous development in the ability to use vision to achieve alignment and to work with two and three spatial features concurrently. Using vision to align objects precisely to other objects and managing more than one spatial relation between an object and a surface are already more elaborated in two year-old humans than in other primates. The human advantage in using hand tools derives in part from this fundamental difference in the relation between vision and action between humans and other primates. [ABSTRACT FROM AUTHOR]

In the Featured Article for this issue of the Journal of Comparative Psychology, Scarf, Johnston, and Colombo (2018) showed that pigeons learned to reproduce (by pecking icons presented on a screen) a four-item serially ordered list without specific training on that list, as macaques did (Figure 1). In Scarf et al.'s (2018) study, all five birds given structured training subsequently mastered multiple sets of two-item, then three-item, and finally four-item lists, with a diminishing training regime for the later four-item lists. Scarf et al.'s (2018) findings have at least two important implications for our science: First, members of evolutionarily distant taxa, pigeons and monkeys, can both produce a serially ordered set of four actions. Second, we are reminded that learning sets can be acquired for diverse tasks--the principles of learning set formation are very general. [ABSTRACT FROM AUTHOR]

This editorial serves as an introduction for the new editorial team of the Journal of Comparative Psychology which is shifting from Chief Editor Josep Call and Associate Editor Irene Pepperberg to Chief Editor Dorothy Munkenbeck Fragaszy and Associate Editor Todd Freeberg. [ABSTRACT FROM AUTHOR]

This feature essay discusses the African striped mice (Rhabdomys) and how closely related-species differ in behavior based on their environment. [ABSTRACT FROM AUTHOR]

The year 2021 marks the 100th year of publication of the Journal of Comparative Psychology by the American Psychological Association. To mark the centennial of the Journal of Comparative Psychology, a series of essays by diverse authors will appear in the four issues of Volume 135. Some of them concern the history of the journal, key figures in its history, and of the discipline; others concern the discipline's current status and its likely near future. [ABSTRACT FROM AUTHOR]

The exploratory or feeding activities of others might influence the timing, the place, or both, of exploratory activities among young group-living individuals, and this influence might affect the information gained by individuals during exploration. This study examined the temporal and spatial aspects of adults' influence on the exploratory behavior of juvenile capuchins, and on the juveniles' acquisition of a novel behavior. Two experimental apparatus, which were initially novel to the juvenile subjects but familiar to the adults, and which provided food when a tool was used properly, were presented to group-housed capuchin monkeys. The apparatus were presented (a) in a central area, in which all animals could inter- act with the apparatus and in which several older group members regularly solved the tasks (group site), and (b) in a protected site within the home cage (crèche) that only juveniles could enter, but from which the rest of the cage, including the group site, could be viewed. Juveniles contacted the apparatus at the crèche more often when there was no apparatus at the group site, but only half the individuals made greater use of the apparatus at the group site than at the crèche when an apparatus was present at both sites. Seven of nine used an apparatus more often when adults also had an apparatus, than when adults did not have an apparatus. These results indicate that juveniles' exploratory activity is only weakly related to adults' activity. The linkage appears closer for young juveniles (20 months or less) than for older juveniles. Moreover, as only older juveniles learned to solve the tasks, coordination of exploration with adults was evidently not related to learning a new skill. [ABSTRACT FROM AUTHOR]

Prehension was examined in tufted capuchins (Cebus apella) and squirrel monkeys (Saimiri sciureus). Individual subjects were videotaped from frontal and sagittal planes while they grasped small objects presented in several ways (in view, out of view, embedded, moving). Capuchins used a precision grip in 30% of trials and in more than half of trials with stationary objects. Most (54%) of the precision grips used were opposition of the thumb to the index finger; however, eight other forms were also observed. Squirrel monkeys never used a precision grip. Data on hand preference, preliminary data on movement velocity, and preliminary observations of movement trajectories (up to the time of hand shaping prior to contact with the object) do not indicate significant differences across genera in these aspects of prehension. The presence of varied precision grips in capuchins and no form of precision grip in squirrel monkeys leads to two conclusions. First, a thumb classification of ‘opposable’ (vs. ‘pseudo-opposable’) is not essential for precision gripping. Capuchins, with pseudo-opposable thumbs, use precision grips routinely. Second, the fundamental difference between these genera, which accommodates precision gripping in capuchins, is the capacity in capuchins (but not in squirrel monkeys) to produce lateral pressure between opposing digits. The anatomy of the spinal pyramidal tract and neuromuscular interfacing in the hands may be more sensitive gauges of manual dexterity than the anatomy of the thumb. [ABSTRACT FROM AUTHOR]

With this issue, the editors inaugurate the Featured Article Essay in the Journal of Comparative Psychology. This brief essay, written by one or both of the editors, highlights one of the articles in each issue that is found to be particularly important, interesting, or innovative. The editors' choice for this issue is the article by Pellis and Pellis (2018) about play fighting in gray mouse lemurs (Microcebus murinus). [ABSTRACT FROM AUTHOR]

The transition from occasional to obligate bipedalism is a milestone in human evolution. However, because the fossil record is fragmentary and reconstructing behaviour from fossils is difficult, changes in the motor control strategies that accompanied this transition remain unknown. Quadrupedal primates that adopt a bipedal stance while using percussive tools provide a unique reference point to clarify one aspect of this transition, which is maintaining bipedal stance while handling massive objects. We found that while cracking nuts using massive stone hammers, wild bearded capuchin monkeys (Sapajus libidinosus) produce hammer trajectories with highly repeatable spatial profiles. Using an uncontrolled manifold analysis, we show that the monkeys used strong joint synergies to stabilize the hammer trajectory while lifting and lowering heavy hammers. The monkeys stringently controlled the motion of the foot. They controlled the motion of the lower arm and hand rather loosely, showing a greater variability across strikes. Overall, our findings indicate that while standing bipedally to lift and lower massive hammers, an arboreal quadrupedal primate must control motion in the joints of the lower body more stringently than motion in the joints of the upper body. Similar changes in the structure of motor variability required to accomplish this goal could have accompanied the evolutionary transition from occasional to obligate bipedalism in ancestral hominins. [ABSTRACT FROM AUTHOR]

Presents a letter to the editor about tool use by capuchin monkeys.